System and method for distortion analysis

ABSTRACT

A method, circuit and system for determining at least one of an amplitude and a relative phase of a signal under test. A reference signal is generated based, at least in part, upon the at least one of the amplitude and the relative phase of the signal under test. The reference signal is combined with the signal under test to generate a residual signal indicative of a distortion within the signal under test. The residual signal is measured.

RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 11/774,774 filed 9 Jul. 2007, the entire contentsof which are incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to analyzing distortion and noise in a testsignal and, more particularly, to analyzing distortion and noise of asinusoidal wave signal.

BACKGROUND

Three primary methods have conventionally been used for measuring thedistortion imparted by a system under test or components of a systemunder test. The system under test is typically driven by a test signaland provides an output signal. According to one technique, the outputsignal may be passed through a notch filter, which may reject thefundamental frequency of the output signal. The harmonics and noise ofthe remaining residual signal may be measured. However, not only may theuse of a notch filter remove parts of the noise in the output signal,but a separate notch filter must be constructed for every frequency thatneeds to be analyzed.

A second technique uses a phase-locked loop to generate a signal at thefundamental frequency of the output signal. The noise in the outputsignal may be evaluated based on the differences between the outputsignal and the signal generated by the phase-locked loop. Unfortunately,using a phase-locked loop to search for the fundamental frequency of theoutput signal may require a great deal of time. Furthermore, theconstraints on the distortion of the phase-locked loop generated signalmay be very aggressive when the output signal being examined has lowdistortion.

A third technique for analyzing the distortion in an output signal mayinvolve the use of a spectrum analyzer, which may measure the individualfrequency components of the output signal. For an output signal havinglow distortion, the spectrum analyzer itself must have very lowdistortion. The residual noise and distortion of the spectrum analyzermay be too great for analyzing a low distortion output signal.Additionally, measuring the individual frequency components of an outputsignal using a spectrum analyzer may be a slow process.

SUMMARY OF THE DISCLOSURE

In a first implementation, a method of analyzing distortion includesdetermining at least one of an amplitude and a relative phase of asignal under test. A reference signal is generated based, at least inpart, upon the at least one of the amplitude and the relative phase ofthe signal under test. The reference signal is combined with the signalunder test to generate a residual signal indicative of a distortionwithin the signal under test. The residual signal is measured.

One or more of the following features may be included. The signal undertest may be an analog signal under test. Determining at least one of theamplitude and the relative phase of the signal under test may includeproviding the analog signal under test to an analog-to-digital converterto generate a digital signal under test. The reference signal may be ananalog reference signal. Generating the reference signal may includereceiving the digital signal under test. A digital reference signal maybe generated based, at least in part, upon the digital signal undertest. The digital reference signal may be provided to adigital-to-analog converter to generate the analog reference signal.

The reference signal may be essentially out of phase with the signalunder test. Combining the reference signal with the signal under testmay include adding the reference signal and the signal under test.

The reference signal may be essentially in phase with the signal undertest. Combining the reference signal with the signal under test mayinclude subtracting the reference signal from the signal under test.

Measuring the residual signal may include providing the residual signalto an analog-to-digital converter. Providing the residual signal to ananalog-to-digital converter may include amplifying the residual signal.

In another implementation, a distortion analysis circuit includes asampling circuit configured to determine at least one of an amplitudeand a relative phase of a signal under test. A signal generator circuitis configured to provide a reference signal based, at least in part,upon the at least one of the amplitude and the relative phase of thesignal under test. A combining circuit is configured to combine thereference signal with the signal under test to generate a residualsignal indicative of a distortion within the signal under test.

One or more of the following features may be included. The samplingcircuit may include an analog-to-digital converter configured togenerate a digital signal under test. The reference signal may be ananalog reference signal and the signal generator circuit may include adigital-to-analog converter. The signal generator circuit may beconfigured to receive the digital signal under test. A digital referencesignal may be generated based, at least in part, upon the digital signalunder test. The digital reference signal may be provided to thedigital-to-analog converter to generate the analog reference signal.

The reference signal may be essentially out of phase with the signalunder test and the combining circuit may be configured to add the signalunder test and the reference signal. The reference signal may beessentially in phase with the signal under test and the combiningcircuit may be configured to subtract the reference signal from thesignal under test.

The distortion analysis circuit may include an amplifier circuitconfigured to amplify the residual signal. The residual signal may be ananalog residual signal. The distortion analysis circuit may include ananalog-to-digital converter configured to process the analog residualsignal and generate a digital residual signal.

In another implementation, an automatic test system includes an inputport configured to receive a signal under test from a circuit to beanalyzed. A distortion analysis circuit includes a sampling circuitconfigured to receive the signal under test and determine at least oneof an amplitude and a relative phase of the signal under test. A signalgenerator circuit is configured to provide a reference signal based, atleast in part, upon the at least one of the amplitude and the relativephase of the signal under test. A combining circuit is configured tocombine the reference signal with the signal under test to generate aresidual signal indicative of a distortion within the signal under test.

One or more of the following features may be included. The samplingcircuit may include an analog-to-digital converter configured togenerate a digital signal under test. The reference signal may be ananalog reference signal and the signal generator circuit may include adigital-to-analog converter. The signal generator circuit may beconfigured to receive the digital signal under test. A digital referencesignal may be generated based, at least in part, upon the digital signalunder test. The digital reference signal may be provided to thedigital-to-analog converter to generate the analog reference signal.

The reference signal may be essentially out of phase with the signalunder test and the combining circuit may be configured to add the signalunder test and the reference signal. The reference signal may beessentially in phase with the signal under test and the combiningcircuit may be configured to subtract the reference signal from thesignal under test.

The automatic test system may include an amplifier circuit configured toamplify the residual signal. The residual signal may be an analogresidual signal. The distortion analysis circuit may include ananalog-to-digital converter configured to process the analog residualsignal and generate a digital residual signal.

The measuring circuit may include an analog-to-digital converterconfigured to measure the residual signal. The test circuit may includean amplifier configured to amplify the residual signal.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages will become apparent from the description, the drawings, andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a distortion analysis system;

FIG. 2 is a flowchart of a process executed by the distortion analysissystem of FIG. 1;

FIGS. 3-4 are diagrammatic representations of an analog signal undertest, an “in-phase” analog reference signal, and an analog residualsignal; and

FIGS. 5-6 are diagrammatic representations of an analog signal undertest, an “out-of-phase” analog reference signal, and an analog residualsignal.

DETAILED DESCRIPTION OF THE DISCLOSURE

Referring to FIG. 1, there is shown an automatic test system 10 formeasuring the distortion imparted by circuit under test 12. Signalgenerator 14 may apply test signal 16 to circuit under test 12, and theresulting output signal (i.e., signal under test 18) generated bycircuit under test 12 may be provided to input port/terminal 20 ofautomatic test system 10. Since test signal 16 is typically adistortion-free signal, the distortion imparted by circuit under test 12may be determined by comparing test signal 16 and signal under test 18.

As discussed above, circuit under test 12 may be driven by test signal16. Examples of test signal 16 may include, but are not limited to, an 8VAC_(peak-to-peak) 1 kHz sinusoidal signal. The amplitude, frequency,and type of test signal (e.g., test signal 16) applied to circuit undertest 12 may vary depending on the type of circuit being tested (e.g.,circuit under test 12). For example, for some circuit types, a squarewave signal or triangle wave signal may be applied to circuit under test12.

In response to test signal 16 being applied to circuit under test 12,circuit under test 12 may provide an output signal (i.e., analog signalunder test 18), which may have a frequency that corresponds to thefrequency of test signal 16. However, the amplitude and phase of analogsignal under test 18 may vary with respect to test signal 16 due tovarious attenuating/phase-shifting components (not shown) includedwithin circuit under test 12. Further, analog signal under test 18 maybe distorted (with respect to test signal 16) due to various distortioninducing components included within circuit under test 12.

Automatic test system 10 may include an electrical circuit 22, which mayreceive (on input port/terminal 22) analog signal under test 18 fromcircuit under test 12 and may measure the distortion within analogsignal under test 18 without requiring access to test signal 16 andwithout knowing the phase/amplitude of test signal 16.

Referring also to FIG. 2, automatic test system 10 may be configured todetermine 100 at least one of the relevant phase and/or the amplitude ofanalog signal under test 18. In order to facilitate such adetermination, analog signal under test 18 may be directly provided (viapath 24) to sampling circuit 26.

Sampling circuit 26 may include analog-to-digital (ADC) converter 28 andanalog signal under test 18 may be provided to analog-to-digitalconverter 28, which may convert analog signal-under-test 18 to digitalsignal under test 30. Analog-to-digital converter 28 may be e.g., a16-bit, analog-to-digital converter operating at approximately a 1 MHzsampling rate. Digital signal under test 30 may be a single bit serialdata stream.

When setting the sampling rate and bit-resolution of analog-to-digitalconverter 28, various criteria may be considered. For example,consideration may be given to the highest anticipated frequency(f_(max)) of analog signal under test 18. Once the highest anticipatedfrequency (f_(max)) is determined, this highest anticipated frequency(f_(max)) may be doubled to define the Nyquist frequency, and thesampling rate may be set to this doubled frequency (2f_(max)) to ensurealias-free signal sampling. Therefore, the sampling rate andbit-resolution of analog-to-digital converter 28 may be configured basedupon e.g., the anticipated test signal applied to circuit under test 12.Accordingly, other sampling rates and bit-resolutions are considered tobe within the scope of this disclosure.

Input port/terminal 20 of automatic test system 10 may be directlycoupled to sampling circuit 26 (via path 24), thus allowing for thedirect application of analog signal under test 18 to sampling circuit26. Alternatively, analog signal under test 18 may be indirectlyprovided to sampling circuit 26 through combining circuit 32 andamplifying circuit 34. When indirectly providing analog signal undertest 18 to sampling circuit 26, automatic test circuit 10 mayde-energize (i.e., turn off) 102 signal generator 36 and set 104 thegain of amplifier 34 to one. In such a configuration, analog signalunder test 18 may pass through combining circuit 32 and amplifier 34 andbe indirectly provided to sampling circuit 26 in an unmodified format.

Upon analog signal under test 18 being provided 106 (either directly orindirectly) to sampling circuit 26, sampling circuit 26 (in conjunctionwith analog-to-digital converter 28) may generate digital signal undertest 30. Digital signal under test 30 may be indicative of the amplitudeand relevant phase of analog signal under test 18. Specifically, asdigital signal under test 30 is a digital representation of analogsignal under test 18, digital signal under test 30 provides informationto signal generator 36 concerning the amplitude of analog signal undertest 18. In this disclosure, relevant phase is intended to mean thephase of analog signal under test 18 (as received by sampling circuit26). By providing digital signal under test 30 (which is a digitalrepresentation of analog signal under test 18), digital signal undertest 30 provides information to signal generator 36 concerning therelevant phase of analog signal under test 18.

When sampling circuit 26 converts analog signal under test 18 to digitalsignal under test 30, digital signal under test 30 is provided to signalgenerator 36, thus enabling the generation 107 of analog referencesignal 38. Upon receiving digital signal under test 30, signal generator36 may process digital signal under test 30 to generate 110 a digitalreference signal (not shown), which is provided 112 to digital-to-analog(DAC) converter 40. Digital-to-analog converter 40 may generate analogreference signal 36 based upon digital reference signal (not shown).

As discussed above, digital signal under test 30 is a digitalrepresentation of analog signal under test 18. When processing digitalsignal under test 30, signal generator 36 may e.g., smooth digitalsignal under test 30 to remove all or a portion of the distortionpresent within signal 30. As discussed above, distortion may be presentwithin digital signal under test 30 due to various distortion inducingcomponents included within circuit under test 12. For example, ifdigital signal under test 30 is a 1,000 Hz sinusoid that includesconsiderable distortion in the ≧12,000 Hz frequency range, signalgenerator 36 may process digital signal under test 30 bylow-pass-filtering digital signal under test 30 to remove the higherfrequency distortion. As discussed above, since digital signal undertest 30 contains accurate amplitude and phase information with respectto analog signal under test 18, the digital reference signal (not shown)generated by signal generator 36 may be an “in phase”, “correctamplitude”, “essentially distortion free” version of digital signalunder test 30, and, therefore, analog signal under test 18.

As discussed above, signal generator 36 may include digital-to-analogconverter 40. An example of digital-to-analog converter 40 may include,but is not limited to, an AD1955 audio pulse code modulateddigital-to-analog converter manufactured by Analog Devices of Norwood,Mass. The digital reference signal (not shown), which is processed bysignal generator 36 to remove all or a portion of the distortion ofdigital signal under test 30, may be provided to digital-to-analogconverter 40. Digital-to-analog converter 40 may generate analogreference signal 38 based upon the digital reference signal (not shown).

Referring also to FIG. 3, an exemplary waveform of test signal 16 isshown. As discussed above, test signal 16 may be an essentiallydistortion-free 8 VAC_(peak-to-peak) 1 kHz sinusoidal signal. Furtherand as discussed above, analog signal under test 18 may be a distortedversion of test signal 16 due to various distortion inducing componentsincluded within circuit under test 12. Due to variousattenuating/phase-shifting components (not shown) included withincircuit under test 12, the amplitude of analog signal under test 18 maybe reduced by “ΔV” and the phase of analog signal under test 18 may beshifted by “θ” when compared to test signal 16. As discussed above,digital signal under test 30 is a digital representation of analogsignal under test 18; and when processing digital signal under test 30to produce the digital reference signal (not shown), signal generator 36may smooth digital signal under test 30 to remove all or a portion ofthe distortion present within signal under test 30. Digital-to-analogconverter 40 may then generate analog reference signal 38 based upon thedigital reference signal (not shown). As shown in FIG. 3, analogreference signal 38 may be an essentially distortion-free version ofsignal under test 18 having essentially the same frequency andamplitude.

As discussed above, an example of digital-to-analog converter 40 mayinclude, but is not limited to, an AD1955 audio pulse code modulateddigital-to-analog converter manufactured by Analog Devices of Norwood,Mass., such as may be used in connection with digital music devicese.g., CD players and MP3 players. As is known in the art, pulse codemodulated digital-to-analog converters are typically capable ofproducing a very accurate sinusoidal (or other analog) output inresponse to a corresponding serial data stream (e.g., the digitalreference signal; not shown). Additionally, the output range ofdigital-to-analog converter 40 may extend beyond the audible range,e.g., to 200 kHz or greater, thereby expanding the available range offrequencies that may be used in connection with automatic test system10.

Continuing with the above-stated example, signal under test 18 andanalog reference signal 38 may be essentially in phase with one another.Combining circuit 32 may combine 114 analog signal under test 18 andanalog reference signal 38 to provide 116 analog residual signal 42,which may be indicative of the distortion within analog signal undertest 18.

Referring also to FIG. 4 and continuing with the above-stated example,analog reference signal 38 (i.e., an essentially distortion-freesinusoid) may be essentially in phase with analog signal under test 18(i.e., a sinusoid distorted by circuit under test 12). The distortion inanalog signal under test 18 may be depicted as the deviation of analogsignal under test 18 from analog reference signal 38.

Combining circuit 32 may include subtracting circuit 44. Subtractingcircuit 44 may subtract 118 analog signal under test 18 from analogreference signal 38 to provide 116 analog residual signal 42, which isillustrated as the mathematical difference between analog signal undertest 18 and analog reference signal 38. While analog residual signal 42is shown having a relatively small amplitude, this is for illustrativepurposes only and is not intended to be a limitation of this disclosure.Specifically, the amplitude of analog residual signal 42 may varydepending on the level of distortion present within analog signal undertest 18 (i.e., the level of distortion introduced by circuit under test12).

Referring also to FIGS. 5 & 6, depending on the manner in which signalgenerator 36 is configured, analog reference signal 38 may beessentially out of phase (i.e., shifted by) 180° with respect to analogsignal under test 18. In such an embodiment, combining circuit 32 mayinclude an summing circuit 46 that may add 120 analog reference signal38 and analog signal under test 18 to provide 116 analog residual signal42.

Automatic test system 10 may measure 122 one or more characteristics ofanalog residual signal 42. Automatic test system 10 may digitize analogresidual signal 42 by e.g., providing 124 analog residual signal 42 toan analog-to-digital converter (e.g., analog-to-digital converter 28included within sampling circuit 26). Once analog residual signal 42 ismeasured 122, various characteristics of analog residual signal 42 maybe determined, such as the SINAD (i.e.,signal-plus-noise-plus-distortion to noise-plus-distortion ratio) andtotal harmonic distortion (i.e., with respect to analog signal undertest 18).

As analog residual signal 42 may be of comparatively small amplitude,analog residual signal 18 may be amplified 124 by amplifier 34 togenerate amplified residual signal 48, which may be provided toanalog-to-digital converter 28. The gain of amplifier 34 may be basedupon the amplitude of analog residual signal 42, and the variouscharacteristics of analog-to-digital converter 28, for example. Forillustrative purposes, amplifier 34 may have a gain of approximatelyone-hundred. Data concerning residual signal 42, amplified residualsignal 48 and/or the digitized residual signal (not shown) may beprovided to third-party device 50 via output port/terminal 52. Examplesof third-party device 50 may include but are not limited to a personalcomputer, a data collection device and/or a display panel, for example.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. Accordingly, otherimplementations are within the scope of the following claims.

1. A method of analyzing distortion comprising: determining at least oneof an amplitude and a relative phase of a signal under test; generatinga reference signal based, at least in part, upon the at least one of theamplitude and the relative phase of the signal under test; combining thereference signal with the signal under test to generate a residualsignal indicative of a distortion within the signal under test; andmeasuring the residual signal.
 2. The method of claim 1 wherein thesignal under test is an analog signal under test and determining atleast one of the amplitude and the relative phase of the signal undertest includes: providing the analog signal under test to ananalog-to-digital converter to generate a digital signal under test. 3.The method of claim 2 wherein the reference signal is an analogreference signal and generating the reference signal includes: receivingthe digital signal under test; generating a digital reference signalbased, at least in part, upon the digital signal under test; andproviding the digital reference signal to a digital-to-analog converterto generate the analog reference signal.
 4. The method of claim 1wherein the reference signal is essentially out of phase with the signalunder test, and wherein combining the reference signal with the signalunder test includes: adding the reference signal and the signal undertest.
 5. The method of claim 1 wherein the reference signal isessentially in phase with the signal under test, and wherein combiningthe reference signal with the signal under test includes: subtractingthe reference signal from the signal under test.
 6. The method of claim1 wherein measuring the residual signal includes: providing the residualsignal to an analog-to-digital converter.
 7. The method of claim 6wherein providing the residual signal to an analog-to-digital converterincludes: amplifying the residual signal.
 8. A distortion analysiscircuit comprising: a sampling circuit configured to determine at leastone of an amplitude and a relative phase of a signal under test; asignal generator circuit configured to provide a reference signal based,at least in part, upon the at least one of the amplitude and therelative phase of the signal under test; and a combining circuitconfigured to combine the reference signal with the signal under test togenerate a residual signal indicative of a distortion within the signalunder test.
 9. The distortion analysis circuit according to claim 8wherein the sampling circuit includes an analog-to-digital converterconfigured to generate a digital signal under test.
 10. The distortionanalysis circuit according to claim 9 wherein the reference signal is ananalog reference signal and the signal generator circuit includes adigital-to-analog converter, the signal generator circuit beingconfigured to: receive the digital signal under test; generate a digitalreference signal based, at least in part, upon the digital signal undertest; and provide the digital reference signal to the digital-to-analogconverter to generate the analog reference signal.
 11. The distortionanalysis circuit according to claim 8 wherein the reference signal isessentially out of phase with the signal under test and the combiningcircuit is configured to add the signal under test and the referencesignal.
 12. The distortion analysis circuit according to claim 8 whereinthe reference signal is essentially in phase with the signal under testand the combining circuit is configured to subtract the reference signalfrom the signal under test.
 13. The distortion analysis circuitaccording to claim 8 further comprising an amplifier circuit configuredto amplify the residual signal.
 14. The distortion analysis circuitaccording to claim 8 wherein the residual signal is an analog residualsignal, the distortion analysis circuit further comprising ananalog-to-digital converter configured to process the analog residualsignal and generate a digital residual signal.
 15. An automatic testsystem comprising: an input port configured to receive a signal undertest from a circuit to be analyzed; and a distortion analysis circuitincluding: a sampling circuit configured to receive the signal undertest and determine at least one of an amplitude and a relative phase ofthe signal under test; a signal generator circuit configured to providea reference signal based, at least in part, upon the at least one of theamplitude and the relative phase of the signal under test; and acombining circuit configured to combine the reference signal with thesignal under test to generate a residual signal indicative of adistortion within the signal under test.
 16. The automatic test systemaccording to claim 15 wherein the sampling circuit includes ananalog-to-digital converter configured to generate a digital signalunder test.
 17. The automatic test system according to claim 16 whereinthe reference signal is an analog reference signal and the signalgenerator circuit includes a digital-to-analog converter, the signalgenerator circuit being configured to: receive the digital signal undertest; generate a digital reference signal based, at least in part, uponthe digital signal under test; and provide the digital reference signalto the digital-to-analog converter to generate the analog referencesignal.
 18. The automatic test system according to claim 15 wherein thereference signal is essentially out of phase with the signal under testand the combining circuit is configured to add the signal under test andthe reference signal.
 19. The automatic test system according to claim15 wherein the reference signal is essentially in phase with the signalunder test and the combining circuit is configured to subtract thereference signal from the signal under test.
 20. The automatic testsystem according to claim 15 further comprising an amplifier circuitconfigured to amplify the residual signal.
 21. The automatic test systemaccording to claim 15 wherein the residual signal is an analog residualsignal, the distortion analysis circuit further comprising ananalog-to-digital converter configured to process the analog residualsignal and generate a digital residual signal.